The application of protective and decorative coatings in the nature of paint or varnish by electrophoretic deposition now is a large industry. Quite commonly such coating processes are called "electrocoating". U.S. Patent and Trandmark Office Class 204, subclass 181, relfects the large growth of this technology in recent years.
Generally, in an electrocoating process of the type concerned here, one or more cathode workpieces and one or more counterelectrodes (anodes) are maintained for a short period of time in an electrical circuit with a dilute aqueous dispersion of film-forming paint binder, usually pigmented, between them. In most such coating operations a net unidirectional electric current is passed between these electrodes at fair voltage (e.g., 50+V.). Generally such current is rectified ac current. This causes deposition of the coating on the electrodes (workpieces). Most frequently the binder dispersion is maintained as a bath in which the electrodes are at least partially immersed. Other methods for forming the electrical circuit also have been proposed, e.g., by showering the workpieces with continuous streams of the current-carrying aqueous coating dispersion.
Representative of the earliest practical electrocoating is that shown in Allan E. Gilchrist's U.S. Pat. No. 3,230,162 of 1966. More recently cationic or cathodic electrocoating has become popular. U.S. Pat. No. 3,799,854 and many subsequent patents involving the electrodeposition of blocked isocyanate and amino resins onto a cathodic workpiece, and the subsequent curing of the electrodeposited film into a crosslinked structure are representative of such cathodic electrocoating. These very practical techniques employ comparatively low molecular weight, structurally ionized resinous material as their fundamental paint binders for the electrocoating, in contrast to the instant latices made by emulsion polymerization.
That is not to say that the cathodic electrodeposition of synthetic latices has not been proposed before. Representative cathodic electrocoating proposals using synthetic latex binders include those of these U.S. patents: Nos. 3,873,488; 3,882,009; 3,994,792; 3,998,776; 4,017,372; 4,225,406; and 4,225,407.
Synthetic latics made by emulsion polymerization as cathodic electrocoating binders have been regarded as having the inherent potential of possessing a number of desirable characteristics such as high coulombic efficiency, high molecular weight for various properties, crosslinkability if desired, low cost, and versatility of composition.
In the emulsion polymerization of cathodic electrocoating latices appreciable amounts of gels (microgels) often form. This intractable material has very high molecular weight. We have found that formost low gloss applications such gel can be tolerated, but it seriously detracts from the ability to obtain optimum gloss and smoothness, as for a one-coat finish. The presence of gel can be assessed conveniently by casting the film of the latex on an aluminum sheet, extracting with acetone, and noting the weight of undissolved residual polymer.
Advantages of the instant invention over prior synthetic cathodic electrocoating latex compositions include the capacity to produce optimally glossy films in a highly efficient manner; additionally, the electrodeposited films of the inventive compositions appear to have enhanced solvent resistance.